A major trouble to be overcome in the conventional types of subsurface oilfield pumps is technically known as a "gas lock". It occurs when the incoming pressure in the tubing is kept up by the orifice outlet valve, or travelling valve, on the upstroke of the piston, and by the orifice inlet valve, or standing valve, on the downstroke of the piston. This downstroke of the travelling valve gives rise to pressure within the fluid between the travelling and standing valves, and causes the travelling valve to open, thus enabling fluid to pass through the travelling valve or orifice outlet valve.
However, when operating in a well that is producing both oil and gas at the same time, the chamber placed between the travelling valve and the standing valve is often filled with gas, and, due to the compressibility of the gas, the downstroke of the travelling valve may not create enough pressure in the chamber below the aforesaid valve to offset the pressure of the column of fluid standing above the valve. As a consequence, the travelling valve remains closed throughout the downstroke. Therefore, the gas between the standing valve and the travelling valve only compresses and expands at every stroke of the piston, which leads to a pump operating defect known as "gas lock", a state of affairs which may go on indefinitely.
The Brazilian patent application PI 8501271 of Mar. 19, 1985 concerns a system meant to solve the troubles referred to above. It consists of an elongated housing containing a first valve fitted into the bottom end of the housing; a part to drive a travelling valve fitted in the upper end of the housing, said part placed so as to slide lengthways in relation to the housing; and a rotating travelling valve fitted between the first valve and the part that drives the travelling valve. The travelling valve possesses upper and lower ends and a sealing surface against either end; a piston to compress fluids, lying between the first valve and the part that drives the travelling valve; and a means to rotate the travelling valve around its lengthwise axis. Said rotating means is connected to the part that drives the travelling valve and the travelling valve itself, whereby the lengthwise movement of the part that drives the travelling valve causes the travelling valve to rotate. The first valve is actuated by fluid pressure variations, which take place inside the housing, while the travelling valve and the part that drives the travelling valve is operated mechanically.
Regarding performance in the foregoing system, one notes that gas locks, hydraulic chock and sealing defects caused by vibration of pump piston are avoided. The same does not apply to wear, since there is no way of ensuring that particles of dirt may not get into the travelling valve assembly. If this does happen, there may occur serious trouble, not only regarding wear but also locking and breaking thereof. If particles store in the joints, this may be enough to bring about locking. Since it is mechanically operated, considerable forces are exerted upon the helical part, which is the most fragile part in the system.
Positive displacement action pumps are also used in the prior art. Throughout pump discharge, the standing valve remains closed and the piston moves from its furthest position to its closest position relative to the standing valve. When this happens, the piston tends to stay in the same place due to the effect of friction between the piston and the pump body, as well as because of the effect of the counter-pressure created between the travelling and the standing valves, as the pump moves towards the standing valve. At the same time, all the weight of the pump rods are bearing directly on the plug, forcing it to be pushed off the valve seat. This forced opening promptly prevents any gas or vapor lock from taking place.
When the valve opens, the distance between the seat and the plug is limited by a stem that joins the plug to the connection. This distance is calculated beforehand in such a way as to enable the fluid to flow forward of the opening under less resistance.
As soon as the piston gets to the point closest to the standing valve, it acts in the opposite direction, into its initial suction stroke. Again, friction between the piston and the pump body tends to hold the piston back until the plug seals against its seat. This takes place when the relative speed of the fluids at either side of the valve is null, therefore the effect of any erosion upon sealing surfaces is considerably less.
When the travelling valve is closed, the pressure between the travelling and the standing valves is reduced as the piston moves off from the standing valve, until it becomes lower than the pressure in the reservoir. When this occurs, the standing valve opens and lets fluid in from the reservoir into the pump body. Finally, when the piston gets to its point furthest away from the standing valve, the piston moves in the opposite direction and the pumping cycle is repeated.
However, a disadvantage of the aforesaid system is that particles of dirt storage may prevent operation from being the ideal, since the relative movement of any fluid bearing particles of sand in suspension erodes the sealing portions of ball or piston valves (particularly in the case of the travelling valve) because of rubbing by particles of silica which is present in any kind of sand.
Another disadvantage is that it is difficult to make use of existing pistons, since not just any kind of piston may be used, and also there is the end cost of the equipment to be considered.